The present application claims the priority of Japanese Patent Application No. 00-5 0081544, filed on Mar. 23, 2000 in Japan, the entire contents of which are hereby incorporated herein by reference.
1. Field of the Invention
The present invention relates to a driving apparatus, and more particularly to a driving apparatus appropriate for the driving of a movable XY stage, the photo-taking lens of a camera, the projecting lens of an overhead projector, or the lenses of binoculars, and the like.
2. Description of the Related Art
One driving apparatus known in the conventional art comprises an impact-type piezoelectric actuator wherein an engaging member to which a photo-taking lens is mounted is caused to engage with a cylindrical drive member such that a prescribed friction force occurs, and a piezoelectric element is attached to one end of the drive member. For example, FIG. 18 shows the basic construction of a driving apparatus for. adjusting the position of the photo-taking lens of a camera.
The driving apparatus 100 in this drawing comprises a piezoelectric element 101 that comprises an electromechanical conversion element, a cylindrical drive member 102 that is driven by the piezoelectric element 101, an engaging member 103 that engages with the drive member 102 through a prescribed friction force, and a drive circuit 104 that applies a drive voltage to the piezoelectric element 101.
The piezoelectric element 101 expands and contracts in response to the drive voltage applied by the drive circuit 104. One end of the piezoelectric element 101 is fixed to a support member 105 in the direction of expansion or contraction, while the other end is attached to one end of the drive member 102 along the axial direction. The photo-taking lens L comprising the driven object is mounted to the engaging member 103 at a prescribed location, and can move along the drive member 102 in the axial direction.
The drive circuit 104 comprises a waveform generating unit 107 and a power amp 108, as shown in FIG. 19. It inputs to the power amp 108 a drive voltage having a 0-5V sawtooth-type waveform obtained from the waveform generating unit 107, and outputs from the power amp 108 a drive voltage having a 0-10V sawtooth-type waveform.
In the driving apparatus 100 constructed as described above, when a drive voltage having the waveform shown in FIG. 20(a) that has gentle rising edges and steep falling edges (a so-called outward waveform) is repeatedly applied to the piezoelectric element 101 from the drive circuit 104, the engaging member 103 moves in the direction of the arrow (a), which is the outward direction (the direction away from the piezoelectric element 101) due to the expansion and contraction of the piezoelectric element 101. In other words, because the piezoelectric element 101 expands gradually during the gentle rising edge of the drive voltage, the engaging member 103 moves in the outward direction together with the drive member 102, and because the piezoelectric element 101 contracts suddenly during the steep falling edge, even though the drive member 102 moves in the return direction, the engaging member 103 slips relative to the drive member 102 and remains in essentially the same position. As a result, when a drive voltage having the waveform shown in FIG. 20(a) is repeatedly applied to the piezoelectric element 101, the engaging member 103 moves intermittently in the direction of the arrow (a).
When a drive voltage having the waveform shown in FIG. 20(b) that has steep rising edges and gentle falling edges (a so-called return waveform) is repeatedly applied to the piezoelectric element 101 from the drive circuit 104, the engaging member 103 moves in the return direction (the direction toward the piezoelectric element 101) opposite the direction of the arrow (a) due to the expansion and contraction of the piezoelectric element 101. In other words, because the piezoelectric element 101 expands suddenly during the steep rising edge of the drive voltage, even though the drive member 102 moves in the outward direction, the engaging member 103 slips relative to the drive member 102 and remains in essentially the same position, and because the piezoelectric element 101 contracts gradually during the gentle falling edge, the engaging member 103 moves in the return direction together with the drive member 102. As a result, when a drive voltage having the waveform shown in FIG. 26(b) is repeatedly applied to the piezoelectric element 101, the engaging member 103 moves intermittently in the direction opposite the direction of the arrow (a). By applying a drive voltage having the waveform shown in either FIG. 20(a) or 20(b) to the piezoelectric element 101 in this way, the photo-taking lens L can be moved in the outward direction or the return direction.
The drive circuit 104 may have the construction shown in FIG. 21, for example. The drive circuit 104 shown in the drawing comprises a first drive circuit 109 consisting of a slow-charging circuit and a rapid-charging circuit, and a second drive circuit 110 consisting of a rapid-charging circuit and a slow-charging circuit, and carries out the drive control of the drive circuits 109 and 110 through prescribed 0-5V control signals generated by a digital circuit.
In other words, the first drive circuit 109 has a construction in which two switches 111 and 112 are serially connected with a constant-current source 113 such that the constant-current source 113 comes between the switches 111 and 112 with respect to the power supply voltage Vs, while the second drive circuit 110 has a construction in which two switches 114 and 115 are serially connected with a constant-current source 116 such that the constant-current source 116 comes between the switches 114 and 115 with respect to the power supply voltage Vs. The piezoelectric element 101 is connected to both ends of the switch element 112 in the first drive circuit 109 and to both ends of the serial circuit connecting the switch element 115 and the constant-current source 116 in the second drive circuit 110.
In the drive circuit 104 shown in FIG. 21, a slow-charging circuit is formed through the closing of the switch 111 of the first drive circuit 109, and a rapid-charging circuit is formed through the subsequent opening of the switch 111 and the closing of the switch 112. By repeatedly carrying out these switch operations through control signals, a drive voltage having the outward waveform shown in FIG. 20(a) is repeatedly applied to the piezoelectric element 101.
Similarly, a rapid-charging circuit is formed through the closing of the switch 114 of the second drive circuit 110, and a slow-charging circuit is formed through the subsequent opening of the switch 114 and the closing of the switch 115. By repeatedly carrying out these switch operations through control signals, a drive voltage having the return waveform shown in FIG. 20(b) is repeatedly applied to the piezoelectric element 101. In this way, the photo-taking lens L can be moved in both the outward and return directions as a result of the movement of the engaging member 103 in the outward and return directions as described above.
When a driving apparatus comprising an impact-type piezoelectric actuator is applied as the drive source for an optical system such as a camera photo-taking lens, it is preferred that the driving apparatus be as inexpensive and compact as possible. However, with the conventional driving apparatus 100, where the drive circuit 104 shown in FIG. 19 is used, the circuit to generate signals having a sawtooth-type waveform is complex, and therefore the problem arises that the goals of low cost and compactness become difficult to achieve. This problem also arises with the drive circuit 104 shown in FIG. 21 as well, because the constant-current sources 113 and 116 have a complex circuit construction.
An object of the present invention is to provide a driving apparatus through which low cost and compactness may be effectively achieved.
In order to achieve the above object, the present invention may include a driving apparatus, comprising an electromechanical conversion element that expands and contracts through the application of a drive voltage, the electromechanical conversion element includes a first end, a second end, and a resonance frequency, a support member that is fixed to the first end of the electromechanical conversion element in a direction of expansion or contraction of the electromechanical conversion element, a drive member that is fixed to the second end of the electromechanical conversion element in the direction of expansion or contraction, an engaging member that engages with the drive member with a friction force, and a drive circuit that drives the electromechanical conversion element, wherein said drive circuit generates a rectangular waveform drive voltage and applies the rectangular waveform drive voltage to said electromechanical conversion element to cause said electromechanical conversion element to expand and contract at different speeds to thereby move the support member and the engaging member relative to each other.
Using this construction, because the displacement waveform for the expansion and contraction of the electromechanical conversion element is a sawtooth-type waveform even when the drive voltage applied to the electromechanical conversion element comprises a rectangular waveform, the support member and the engaging member can be moved relative to each other due to the fact that the electromechanical conversion element expands and contracts at different speeds. Therefore, using the present invention, because the drive voltage applied to the electromechanical conversion element can have a rectangular waveform, the construction of the drive circuit may be simplified and the driving apparatus may be made smaller and at a lower cost. Furthermore, the relationship between the drive frequency fd and the resonance frequency fr of the electromechanical conversion element to which the support member and drive member are fixed should be expressible by about fd  greater than 0.3xc3x97fr or about fd  less than 1.5xc3x97fr.
The drive voltage may have a duty ratio D that is expressed by about 0.05 less than D  less than 0.45. Using this construction, the engaging member is moved in the outward direction due to the fact that the displacement waveform for the expansion and contraction of the electromechanical conversion element has a sawtooth configuration with gentle rising edges and steep falling edges. As a result, the photo-taking lens or other driven object attached to the engaging member may be moved in the outward direction away from the electromechanical conversion element.
The drive voltage may also have a duty ratio D that is expressed by about 0.55 less than D  less than 0.95. Using this construction, the engaging member is moved in the return direction due to the fact that the displacement waveform for the expansion and contraction of the electromechanical conversion element has a sawtooth configuration with steep rising edges and gentle falling edges. As a result, the photo-taking lens or other driven object attached to the engaging member may be moved in the return direction toward the electromechanical conversion element.